[RASMB] Hydration of DM/OG cont...

Fumio Arisaka farisaka at bio.titech.ac.jp
Wed Nov 26 06:57:00 PST 2003


Dear RASMBers,

Joachim Behlke and Tom Laue sent me the following messages concerning
my previous inquiry, which are highly instructive to me and perhaps to some
of you, too, so I would like to share them with those of you.  I thank them
as well as Ariel and John, and assume both Joachim and Tom do not mind.  Fumio

-----------------------------------------------------------
 From Joachim:

Dear Fumio,
in solution the substances are hydrated, however in many cases
nobody know exactly the number of bound water
molecules.Therefore, it is difficult to determine the concentration
exact. For determination of v-bar using the densitometer the
density of a substance is plotted versus the concentration of the
non-hydrated substance, often derived from specific parameters
(extinction coefficient etc.). In this case you will get v-bar non-
hydrated.
When you measure the density in AUC cells and compare this with
hydrated markers of known density, of cause you will get the
density or v-bar of hydrated molecules.

Regards,

Joachim

-----------------------------------------------------------
 From Tom:

Hi Fumio,
The values of vbar that are determined by density readings relate to how 
the concentration of the solute is determined. There is a good discussion 
of this in Heine Eisenberg's 2000 paper on a Gibbsian perspective of 
analytical ultracentrifugation.
Ultimately, the molecular weight calculated from sedimentation data refers 
to the form of the material used to measure the concentration in the vbar 
determination. For example, if you measure the vbar for DNA in NaCl, then 
do sedimentation in concentrated CsCl, the molecular weights you calculate 
will be for the Na-DNA. In the case of proteins and the vbar used in 
calculations come from data obtained (or calculated using Traube's rule) 
for the anhydrous amino acids. Hence, the molecular weights obtained are 
for the anhydrous protein.
Best wishes,
Tom
-----------------------------------------------------------
Thanks, Tom.  I once read Dr. Henryk Eisenberg's paper, but I need to read 
it again in this context.


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